HEVC/H.265: Everything you need to know

Cunning H.265

The successor to h.264 is, cunningly, h.265, an ITU standard just a fraction away from being approved at the time of writing that's based on the High Efficiency Video Coding (HEVC) system. As we’ve seen, the complexity and therefore effectiveness of video codecs is largely controlled by the performance of the devices on which they will be replayed. HEVC leverages improvements in the performance of consumer electronics, aiming to achieve the same image quality at half the bitrate of h.264 through the application of more advanced image-encoding techniques, while being no more than three times harder to decode than High-profile h.264. On the face of it, this seems like a questionable deal, with a tripling of requirements while only doubling performance, but with Arm and Intel currently competing quite effectively to create electronics that do more work for less money and less electricity, this seems like a rather ungenerous criticism.

Achievements in improving the performance of compression codecs have been mainly regulated by the performance of the consumer devices that must decode and display content, and recent improvements in this area have made ever more complex and effective codecs feasible. HEVC is recognisably a development of h.264, and as the overview to the specification states, there is no one major change that accounts much more than any other for the improvement in performance.

Variable block size

This is inevitably a very partial and incomplete discussion as the complete HEVC specification is neither short nor simple, but perhaps most fundamentally, HEVC does not start by breaking the image up into squares of equal size, as had been the case in previous standards. Instead, it has the flexibility to select the block size to maximise the effectiveness of its other techniques, depending on the image content. The block size itself is, optionally, larger – up to 64 pixels square, as opposed to 16 – which makes for additional efficiencies, especially on the more CPU-intensive profiles of HEVC which require the use of larger blocks. Beyond this, HEVC has the option to break these larger blocks down into smaller ones, treating the smaller blocks individually with regard to the encoding techniques that are used to compress them.